Accessory lens having at least one bravais point outside the lens



350-422 SR OR 3.4 1.338

auuwn mu April 29, 1969 J. R. JOHNSON ETAL 7 3,441,338 ACCESSORY LENS HAVING AT LEAST ONE BRAVAIS POINT OUTSIDE THE LENS Filed June 30, 1965 FIG I JAMES R- JOHNSON JAMES E- HARVEY RUDOLF KINGSLAKE INVENTORS ATTORNEYS United States Patent 3,441,338 ACCESSORY LENS HAVING AT LEAST ONE BRAVAIS POINT OUTSIDE THE LENS James R. Johnson, James E. Harvey, and Rudolf Kingslake, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed June 30, 1965, Ser. No. 468,265

Int. Cl. G02b /02, 9/06; G03h 21/00 US Cl. 350-183 7 Claims ABSTRACT OF THE DISCLOSURE An accessory lens, having each of its principal points closer to its respective focal point than to the other focal point and having at least one of its principal points within the outside surfaces of the lens, is disclosed for use in an optical system to change the magnification of a primary lens in the system.

This invention relates to accessory lenses and their uses. More specifically, this invention relates to the design and use of an accessory lens to change the magnification of a photographic printing optical system without the need for refocusing the principal optical system.

A commercial photographic printer is often designed to work at specific conjugates for a specific magnification. With such a printer, if it is desired to work at a different magnification, as with a different size original, a complete new optical system must be used.

It is an object of this invention to provide a method for conversion of such a photographic printer to operate at a different magnification without the need for refocusing the basic optical system or moving either conjugate point.

It is another object of this invention to provide a wellcorrected accessory lens that will allow such a conversion without introducing substantial aberrations into the overall printing optical system.

The former of the above objects is accomplished by placing an accessory lens of proper design in front of the original at a position such that the accessory lens creates an enlarged virtual image of the original in approximately the same plane as the original. The latter object is accomplished by foregoing traditional designs of such accessory lenses and instead designing them in the form of thick positive magnifiers.

In the drawings:

FIG. 1 is a diagrammatic axial cross section of a lens constructed and applied according to the invention in a manner to increase magnification;

FIG. 2 is a diagrammatic axial cross section of a lens I applied according to the invention in a manner to de crease magnification;

FIGS. 3 and 4 are diagrammatic axial cross sections of other accessory lenses constructed according to the invention.

For purposes herein the term component shall include a combination of elements cemented together or separated by only a small weak airspace and, of course, a single element not part of such a combination.

According to 'FIG. 1 an unusually small object 9 is placed in a plane for projection by a primary focusing means, in this case a printing lens 6. The printing lens 6 and its image format are such that the format will be properly filled only with an object the height of arrow 10. An accessory lens I designed according to the invention is inserted at a position such that it forms a virtual'image of object 9 the height of arrow 10 in the same plane as object 9. The printer will then work at 3,441,338 Patented Apr. 29, 1969 "Ice the same conjugates as before with no refocusing'of the printing lens 6 but at an increased magnification.

According to FIG. 2, if it is desirable to reduce the magnification, the accessory lens can be placed in front of the image plane in a similar manner. What would ordinarily be an image the height of arrow 12 is reduced to one the height of arrow 11.

The following is an explanation of the basic principles involved in such a lens which will aid in further understanding the part of this invention dealing with its design.

It has been pointed out in the optical literature that for each lens system there are two object positions where the image of the object lies in the same plane as the object. It is believed that this principle was first pointed out in Bravais, Annales de chimie et de physique @1851), vol. 33, p. 494. These positions have become known as Bravais points.

'For a Bravais point to be useful in its application in this invention, it must be outside of the lens itself. Its position with any lens is dependent upon the location of the principal points of the lens relative to the focal points. 'In lenses in which the principal points coincide, the Bravais points also coincide at the principal points. In systems in which each principal point lies closer to the opposite focal point than to its own focal point, i.e., systerns in which the two focal lengths overlap, the Bravais points lie between the principal points. This type of lens includes all negative lenses and many positive systems, including most of those of the Petzval type.

If each principal point lies between its respective focal point and the other principal point, as is normal for a biconvex lens, the Bravais points will lie outside of the principal points. In this case the wider apart the principal points of the lens, the closer the Bravais points will be to focal points of the lens and the higher will be the magnification at the Bravais point.

The position of the Bravais point for any lens system and the magnification at such point can be determined algebraically with the following formulas:

where p and p are the distances from the Bravais point to the respective principal points, Z is the distance between the principal points, and f is the focal length of the lens.

Thus, in order to place a Bravais point outside of the lens, a lens designer must design either a lens which places one or both of the principal points outside of the lens or one in which the principal points (with nonoverlapping focal lengths) are sufficiently wide apart to push a Bravais point outside the lens.

The original lens suggested by Bravais and all known lenses using this principle prior to this invention have solved this problem by using a widely spaced combination I of thin lens components of opposite powers. This combi- We have obtained much better results than those previously obtained by designing Bravais lenses in which at least one of the principal points is inside the lens and the other principal point is either inside the lens or relatively close to it. More specifically, in all of our examples at least half of the distance between the principal points is inside of the lens. The best corrections were obtained when both principal points were inside of the lens.

Consistent with this invention, the preferred design which places at least one Bravais point outside of the lens and gives the best corrections is the thick positive magnifier. It has fairly widely separated principal points which principal points lie closer to their respective focal points than to the opposite focal points. When designed using all positive components, it can be sutficiently well corrected to be used in high quality color printing work. If more than one component is used, the airspace separating them is preferably of positive power.

Such a magnifier is capable of excellent corrections for spherical aberration, chromatic aberration, flatness f field and distortion. The distance at which such an accessory lens is placed from the object can be varied according to the placement of the principal points and will depend upon the physical requirements of the system and the magnification desired.

FIGS. 1, 3 and 4 show specific embodiments of the invention which are well corrected. In these figures and in all examples and claims below, from front to rear, the components are numbered from I to II, the elements are numbered from 1 to 4, the radii of curvature from R to R and the thicknesses from t to 11;. S is theseparation from the rear surface of the accessory lens to the object, and S is the separation between components of the lens, if any. The indexes of refraction N are for the D line of the spectrum and V is the dispersive index.

Well-corrected Bravais magnifiers suitable for use according to the invention can be constructed according to the following examples:

EXAMPLE 1 (FIG. 1) Magnification=1A0 Focal length=100 mm.

v In this example the preferred Bravais point is 17.5 mm. o

behind R and each principal point is inside the lens.

EXAMPLE 2 (FIG. 1)

Magnification=L Focal length=l00 mm.

Component Element N V Ill-R h-h in mm. in mm.

R1=+367 1 1.517 64.5 ti=8.6 I Rz= -28.8

In this example the preferred Bravais point is 16.7 mm. behind R and each principal point is inside the lens.

EXAMPLE 3 (FIG. 3) Magnification=L70 Focal length= mm.

In this example the preferred Bravais point is 8.65 mm. behind R and both principal points lie inside the lens.

EXAMPLE 4 (FIG. 4)

Magnification=1.55 Focal length=100 mm.

Component Element N V Iii-Rs M44 in mm. in mm.

R +93.5 1 1. 526 54. 6 ti=38.3 I R 59.4

2 l. 689 30. 9 ta=5.7

S =1.4 R =+380 3 1. 689 30. 9 t =5.7 II R5 +472 In this example the preferred Bravais point is 5.75 mm. behind R and both principal points are inside the lens.

It should be noted that in Examples 2 and 3 the lens is placed so that the object and image in question are slightly displaced from each other. This is done to compensate for a glass plate which is removed when the lens is substituted. For purposes of describing and claiming this invention, it should be understood that such a variation does not prevent the object and image from being considered in substantially the same plane.

The invention has been described in considerable de tail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

We claim: 1. In an optical system having 1) two conjugate points, (2) a primary imaging means for producing an image at one of said conjugate points of an object placed at the other conjugate point, I (3) an accessory lens placed relative to one of said conjugate points such that it will produce a virtual image at substantially said conjugate point of an object placed at substantially said conjugate point, the improvement wherein said accessory lens has constructional characteristics defining two principal points, such that (a) each of its principal points lies closer to its respective focal point than said principal point lies to the other focal point, and (b) at least one-half of the distance between said principal points lies between the outside surfaces of said accessory lens.

2. In an optical system having (1) two conjugate points, (2) a primary imaging means for producing an image at one of said conjugate points of an object placed at the other conjugate point,

(3) an accessory lens to placed relative to one of said conjugate points that it will produce a virtual image at substantially said conjugate point of an object placed at substantially said conjugate point,

the improvement wherein said accessory lens has constructional characteristics defining two principal points, such that (a) each of said principal points lies closer to its respective focal point than such principal point lies to the other focal point, and (b) at least one of its principal points lies between the outside surfaces of said accessory lens.

3. The improvement according to claim 2 wherein both said principal points lie between the outside surfaces of said accessory lens.

4. The improvement according to claim 1 wherein all components of said accessory lens are of positive power.

accessory lens is two positive components separated by a positive airspace.

References Cited UNITED STATES PATENTS 2,961,924 11/1960 Muller 350-225 DAVID SCHONBERG, Primary Examiner.

RONALD J. STERN, Assistant Examiner.

US. Cl. X.R. 88-24; 350-231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,441,338 April 29, 1969 James R. Johnson et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 47, "p=p'z" should read p'=p-z line 50 "m= should read m=p Column 3, line 51 p Y 't =2S" should read t1=27.9 line 64, "Magnification 1.20" should read Magnification 1.22 line 74, "16.7mm should read 18.0mm. Column 4, line 24, "Magnification 1.55" should read Magnification 1.58 line 38, "5 75mm. should read 6 72mm Column 5 line 3, "to", first occurrence, should read so Signed and sealed this 23rd day of February 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

